Why are the Cycles/BI Voronoi Procedural Textures Different?
The Voronoi Texture used by the displacement modifier is generated differently since it can also be used in the Blender Internal renderer.
The node you speak of is the Cycles texture node, a completely different engine and thus completely different parameters and code.

But yeah, I'd like to see some more procedural textures ported from BI to Cycles, particularly this Voronoi texture and the Cloud texture (which has many different algorithms to choose from, whereas Cycle's Noise texture is a simple Perlin noise if I remember right)